By default (with no way to change it, apparently), Safari will open a new window when opening external application links (like from Mail or TextEdit), instead of reusing the same window or opening a tab. However, if the current tab is blank -- i.e., you've just opened it with no web page displayed -- Safari will use that tab instead of opening a new window. A little bit of a time saver, as opposed to consolidating windows by shuffling around the URLs.

Just set in the general preferences:
"Open links from applications -> in the current window"
Then Safari will open the Page in a new Tab. In other words,
the page you have open will not get overwritten with any of
the settings. cool.

Another useful thing that I read somewhere is to put a folder of
your link in the bookmark bar. Then when you click on it at the
end of the menu it'll say open in tabs, which will open them all up
quickly.

Doing it this way is a little faster because you do not have to go
show your bookmarks first.

Also, try cmd-clicking on the bookmark in the bookmark bar to
and you'll get "Open in Tabs" as your first option. Even faster
then clicking to bring up the bookmarks in a menu.

Maybe it's me, but Command-clicking on items in the Bookmark
bar does not produce the described result. Control-clicking on
either an individual item or a folder in the bookmark bar will in
fact produce a contextual menu popup with various options,
including Open In New Tab, Open in New Window, Edit Name,
Delete, etc. Command-clicking on an individual item will open it
in a new tab, and command-clicking on a folder will open all of
its constituent URLs in separate tabs, first replacing any current
tabs you have open. I really wish that some sort of modifier-key
could be made to open some options on the bookmark icon in
the bookmark bar so that I could work with it without having to
replace the contents of the current tab.